1. Field of the Invention
The present invention relates to an image forming apparatus relying on electrophotography, and more particularly to how to measure the amount of toner adhered to a toner carrying member that carries it.
2. Description of Related Art
In an image forming apparatus employing an electrophotographic process, density correction and color-shift correction are generally achieved by transferring toner directly onto a toner carrying member to form a patch image (reference image) thereon and then detecting the amount of toner with which and the position at which the patch image is formed. For example, in a tandem-type full-color image forming apparatus, patch images of different colors are formed on a transfer belt by a cyan, a magenta, a yellow, and a black image forming section, and the thus formed patch images are then detected by detecting means for the purposes of density correction and color-shift correction.
Generally used as the detecting means is optical detecting means realized with a combination of a light—emitting device such as an LED and a light—receiving device such as a photodiode. For example, to measure the amount of toner adhered to the transfer belt, measurement light is shone from the light-emitting device on a toner image. The measurement light is then reflected partly on the toner and partly on the belt surface so as to be received by the light-receiving device.
When the amount of toner adhered is large, more light is intercepted by the toner, and accordingly less light is reflected on the belt surface; thus the light-receiving element receives less light. By contrast, when the amount of toner adhered is small, more light is reflected on the belt surface, and thus the light-receiving element receives more light. Thus, by monitoring the density of the patch images of the different colors as known from the output level of the light reception signal that is so outputted as to be commensurate with the amount of reflected light received, then comparing it with a prescribed reference density, it is possible to adjust the charge voltage, the development bias voltage, and the amount of light emitted from an LED head and thereby achieve density correction for each color.
To perform precise density correction based on patch images, the amount of toner adhered to the transfer belt needs to be measured precisely. For example. according to Japanese Patent Application Laid-open No. 2002-23433 (hereinafter, patent publication 1), measurement light is shone on a reference patch image formed on a toner carrying member, and the amount of light regularly reflected therefrom is detected to measure the amount of toner adhered. Here, if the toner carrying member has a low-gloss surface, regardless of the patch density, the sensor output is low, making it difficult to perform precise detection of the patch density. For this reason, patent publication 1 recommends the use of a toner carrying member having a specific surface gloss or higher (specifically 50 or more but 98 or less when measured at an angle of 20°).
On the other hand, according to Japanese Patent Application Laid-open No. 2001-194843 (hereinafter, patent publication 2), measurement light is shone on a reference image, and the difference between the amount of light regularly reflected therefrom and the amount of light irregularly reflected therefrom is detected to measure the amount of toner adhered. According to this method, unlike the method disclosed in patent publication 1 according to which only the amount of light regularly reflected is detected, the sensor output varies greatly with the patch density regardless of whether the toner is black or colored, permitting precise detection of the amount of toner adhered in particular in a color image forming apparatus. Even in this case, to obtain a sufficient amount of regularly reflected light, as recommended in patent publication 1, a toner carrying member having a specific surface gloss or higher is used.
Generally, as an image forming apparatus is used for an extended period of time, and also under the influence of ingredients (for example, an abrasive component) of toner other than the toner component, the surface condition of a toner carrying member changes. This makes it impossible to keep the initial surface condition until the expiration of the warranted normal-operation period of the toner carrying member. This change in the surface condition causes the light reception output of the light-receiving device to vary. Thus, according to the methods disclosed in patent publications 1 and 2, it is difficult to precisely measure the amount of toner adhered.
To overcome this, there has been proposed a method for precisely measuring the amount of toner adhered regardless of change in the surface condition of a toner carrying member. Specifically, Japanese Patent Application Laid-open No. 2004-177608 (hereinafter, patent publication 3) discloses an image forming apparatus as shown in FIGS. 5A to 5C that uses a toner adhesion amount measurement apparatus (see Japanese Patent Registered No. 2729976 (hereinafter, patent publication 4)) provided with light emitting means for emitting, at a predetermined angle, uniformly polarized light, a polarization splitting prism for splitting the light reflected from a toner carrying member or from toner into light polarized in the same way as the emitted light and light polarized otherwise, and first and second light receiving means for receiving light polarized in the two different ways, respectively, thus split by the polarization splitting prism. Here, the image forming apparatus is so adjusted that the levels of the reception output signals as obtained from the first and second light-receiving means when a predetermined amount of toner is adhered to the toner carrying member are equal.
The toner adhesion amount measurement apparatus (optical detecting means) 9 shown in FIGS. 5A to 5C includes a light-emitting device (for example, an LED) 20 that shines measurement light on the surface of a transfer belt 8 and a first and a second light-receiving device 21 and 22 that receive the light reflected from the transfer belt 8. Between the light-emitting device 20 and the transfer belt 8, a polarization filter 23 is arranged that transmits only P-polarized light. On the other hand, between the second light-receiving device 22 and the transfer belt 8, a polarization separation prism 24 is arranged that transmits P-polarized light to direct it to the first light-receiving device 21 and that reflects S-polarized light to direct it to the second light-receiving device 22.
Suppose now that a sufficient amount (proper amount) of toner has been transferred onto the transfer belt 8, and measurement light is shone from the light-emitting device 20 on the transfer belt 8. Then, as shown in FIG. 5A, of the P-polarized light (hereinafter referred to as regularly reflected light) P1 and the S-polarized light (hereinafter referred to as irregularly reflected light) S1 contained in the measurement light, the light S1 is intercepted by the polarization filter 23, and thus only the light P1 passes through the polarization filter 23 to reach the transfer belt 8. Since the light P1 is not transmitted through the toner t, it does not reach the surface of the transfer belt 8, but is reflected on the surface of the toner t.
The reflected light is separated by the polarization separation prism 24 into regularly reflected light P3 and irregularly reflected light S3, of which the light P3 is received by the first light-receiving device 21 and the light S3 is received by the second light-receiving device 22. The first and second light-receiving devices 21 and 22 perform photoelectric conversion on the light they have received, and output a first and a second output signal, respectively. The first and second output signals are then subjected to A/D conversion, and are then fed to a control section (unillustrated). The control section adjusts the output levels (gains) of the first and second light-receiving devices in such a way that, when a proper amount of toner is adhered to the transfer belt 8, the levels of the first and second output signals are equal.
By contrast, suppose now that, as shown in FIG. 5B, no toner image has yet been formed on the transfer belt 8, and that measurement light containing regularly reflected light P1 and irregularly reflected light S1 is shone on the transfer belt 8. Then, the light S1 is intercepted by the polarization filter 23, and only the light P1 reaches the surface of the transfer belt 8, on which the light P1 is reflected as reflected light containing varying proportions of regularly and irregularly reflected light according to the surface shape (for example, surface roughness) of the transfer belt 8. The reflected light is then separated by the polarization separation prism 24 into regularly reflected light P2 and irregularly reflected light S2, of which the light P2 is received by the first light-receiving device 21 and the light S2 is received by the second light-receiving device 22.
The first and second light-receiving devices 21 and 22 perform photoelectric conversion on the light (P2 and S2) they have received, and output a first and a second output signal, respectively. The first and second output signals are then subjected to A/D conversion, and are then fed to the control section. The control section sets, as a reference value, the difference between the first and second output signals at the moment. After the adjustment of the output levels of the first and second light-receiving devices and the setting of the reference value as described above, the amount of toner adhered to the transfer belt 8 is measured as shown in FIG. 5C.
In FIG. 5C, as in FIGS. 5A and 5B, of the P-polarized light P1 and the S-polarized light S1 contained in the measurement light, the light S1 is intercepted by the polarization filter 23, and only the light P1 reaches the toner. If the amount of toner with which the toner image formed on the transfer belt 8 is formed is insufficient, part of the light P1 that has struck the toner is reflected on the surface of the toner t, and the rest is transmitted through the toner t. The light that is transmitted through the toner t is then reflected on the surface of the transfer belt 8.
That is, the light P1 that has reached the surface of the transfer belt 8 is reflected partly as regularly reflected light P2 and partly as irregularly reflected light S2. The regularly and irregularly reflected light P2 and S2 is then separated by the polarization separation prism 24, so that the light P2 is received by the first light-receiving device 21 and the light S2 by the second light-receiving device 22. Likewise, the regularly and irregularly reflected light P3 and S3 reflected on the surface of the toner t is separated by the polarization separation prism 24, so that the light P3 is received by the first light-receiving device 21 and the light S3 by the second light-receiving device 22.
As described above, the first and second light-receiving devices 21 and 22 performs photoelectric conversion on the light they have received, and output the first and second output signals, respectively, which are then subjected to A/D conversion, and are then fed to the control section. The control section calculates, as a measured output value, the difference between the first and second output signals, and then, based on the above-mentioned reference value, corrects the measured output value to determine a corrected output value. Thus, if the corrected output value determined when no toner is adhered equals 1, the corrected output value at a given moment is calculated as the measured output value divided by the reference value.
The control section has stored therein, as toner adhesion amount data, the relationship between the measured output value and the amount of toner adhered. Thus, according to the corrected output value, and based on the toner adhesion amount data, the control section can know the amount of toner adhered (image density) and output it as a result of measurement. The toner coverage ratio C is calculated according to formula (1) below.C=1—[(P−P0)—(S−S0)]/[(Pg−P0)−(Ss−S0)]  (1)
where                P represents the light reception output voltage corresponding to the amount of light regularly reflected from the reference image;        S represents the light reception output voltage corresponding to the amount of light irregularly reflected from the reference image;        P0 represents the light reception output voltage corresponding to the amount of light regularly reflected when no light is shone;        S0 represents the light reception output voltage corresponding to the amount of light irregularly reflected when no light is shone;        Pg represents the light reception output voltage corresponding to the amount of light regularly reflected from the surface of the toner carrying member; and        Sg represents the light reception output voltage corresponding to the amount of light irregularly reflected from the surface of the toner carrying member.        
That is, when a proper amount of toner is adhered to the belt, the output levels (gains) of the light-receiving devices are adjusted such that P−P0=S−S0, and hence the coverage ratio C equals 1. When no toner is adhered to the belt, P=Pg and S=Sg, and hence the coverage ratio C equals 0. In a case where the amount T of toner adhered is 1 mg/cm2 when the coverage ratio C equals 1, the amount T of toner adhered is calculated directly according to formula (1) noted above.
According to the method disclosed in patent publication 3, when a predetermined amount of toner is adhered, the difference between the light reception output signals from the first and second light-receiving devices 21 and 22 remains 0 regardless of the surface condition of the toner carrying member. Thus, the levels of the light reception output signals can be corrected according to the change, as results from secular change, in the surface condition of the transfer belt 8. This permits precise measurement of the amount of toner adhered.
As the surface gloss of the toner carrying member secularly changes, however, the relationship between the coverage ratio C calculated according to formula (1) above and the amount of toner adhered changes. FIG. 6 shows the results of measurement of the coverage ratio and the toner adhesion amount as measured by the method described above with respect to two types of transfer belt A and B having an equal surface gloss (60 at a measurement angle of 60° and 13 at a measurement angle of 20°) plus another transfer belt C, which is the same as the transfer belt A but has undergone an endurance test for a predetermined number of hours to have a lower surface gloss (2 at a measurement angle of 60° and 0 at a measurement angle of 20°). The surface gloss was measured on a gloss checker (the model IG-330) manufactured by HORIFBA Ltd.
The transfer belt A had a brown color expressed as (17, 8, 5) in L*a*b* notation (whereby a given color is defined in a color space assumed along three mutually perpendicular axes, namely the L* axis representing lightness, the a* axis representing red-to-green chromaticity, and the b* axis representing yellow-to-blue chromaticity), or as (59, 41, 38) in RGB notation (whereby a given color is defined in terms of the intensities of light of three, namely R (red), G (green), and B (blue), colors). Likewise, the transfer belt B had a whitish light brown color expressed as (76, 4, 20) in L*a*b* notation, or as (209, 182, 149) in RGB notation, and the transfer belt C had a gray color expressed as (44, 0, −7) in L*a*b* notation, or as (97, 104, 114) in RGB notation.
As will be clear from a comparison between the results with the transfer belt A (indicated by a solid line in the figure) and those with the transfer belt B (indicated by a dotted line in the figure), so long as the surface gloss is equal, two different colors, like brown and whitish light brown in this particular case, exhibit an approximately identical relationship between the coverage ratio and the toner adhesion amount. By contrast, as will be clear from a comparison between the results with the transfer belt B and those with the transfer belt C (indicated by a dash-and-dot line in the figure), if the surface gloss differs greatly, even two similar colors, like whitish light brown and gray in this particular case, exhibit different relationships between the coverage ratio and the toner adhesion amount, except when a proper amount of toner is adhered and hence the coverage ratio equals around 1.
Thus, to keep an identical relationship between the coverage ratio and the toner adhesion amount throughout the whole range, even with the method according to patent publication 3, it is necessary to maintain the gloss of the toner carrying member. As described earlier, however, it is difficult to maintain the initial surface condition until the expiration of the warranted normal-operation period of the toner carrying member. Moreover, when the transfer belt 8 is formed of a hard material, stress concentration between the transfer belt and the photoconductive member causes toner to adhere firmly to the photoconductive member non-electrostatically. As a result, even when a transfer voltage is applied to the transfer belt or the transfer roller, the toner on the surface of the photoconductive member cannot completely be transferred to the transfer belt. This increases the likeliness of density loss in a central portion of a solidly colored area.
Accordingly, it is preferable to use, as the transfer belt 8, an elastic belt formed of a soft material such as rubber with a view to alleviating stress concentration. In general, however, as compared with hard belts, elastic belts are more prone to deterioration in surface condition, and more quickly lose gloss. Moreover, as the materials for soft belts, only limited kinds of materials are satisfactory in performance. Thus, it is extremely difficult to maintain gloss.